As is known, sound-absorbing panels for ceilings and walls have been provided in the past in which a membrane is mounted on a sound-damping porous layer of glass fibers, rock wool or a honeycomb structure. In one type of prior art sound-absorbing panel of this type, a membrane which covers a honeycomb structure is porous such that the sound waves can pass through the membrane and effectively become trapped within the honeycomb cells. These are effective over a relatively wide range of sound frequencies, including the higher frequencies. Other types of prior art panels use non-porous membranes in combination with glass fiber or rock wool backings, but these are severely limited in their high frequency response.
While effective at the higher frequencies, one difficulty with panels employing porous membranes is that they cannot be used for noise reduction in rooms requiring a high degree of sanitation. That is, porous sound absorbers can easily become contaminated with undesirable pollutants such as water, oil, dirt and the like. More importantly, the porous sound absorbers provide a place for fungus, mold, bacteria and other undesirable living organisms. Such conditions cannot be tolerated in highly sanitary environments such as food-processing plants, breweries, soft-drink plants and the like.
Heretofore, many different proposals have been advanced in an effort to make porous materials suitable as sound absorbers in sanitary environments. Commonly, these include covering the porous material with thin, non-porous polymeric membranes such as Mylar, Tedlar or polyethylene. While these membranes provide some protection for the porous material, they degrade its acoustical performance, especially at higher frequencies. Also, these protective materials must be relatively thin so as not to seriously degrade the sound-absorption characteristics of the porous material. This requirement makes these materials susceptible to puncture and, hence, exposure of the porous material to the surrounding environment. As a consequence, current FDA and USDA requirements preclude their use in sanitary environments.
Another class of sound absorber which has been proposed for sanitary environments utilizes cells or slots in a specific geometry to achieve noise reduction. While these cells or slots can be covered with a porous layer, their sound-reducing properties are seriously degraded if they are covered with a non-porous layer. Another disadvantage of these cellular and slotted absorbers is that the cells and slots can provide areas for organic growth and are not easily cleanable. Furthermore, cellular and slotted absorbers are relatively expensive and heavy so as to be undesirable for use as ceiling tiles.